What is superheat, and what does it tell about a system’s operation? The term superheat generally is used to describe the state of a refrigerant. Refrigerant in its vapor state is said to be superheated because it is at a temperature above its saturation (boiling point) temperature. The degree at which a refrigerant is superheated is the temperature difference between its actual temperature and its saturation temperature. For example, if the saturation temperature of a refrigerant is 20°F and its actual temperature is 30°, then the refrigerant is said to be superheated by 10°. This could be also stated as “the refrigerant has 10° of superheat.”
Between the Evaporator and Condenser
In a properly operating system, the refrigerant will be in a superheated state from the last section of the evaporator to the first section of the condenser. Technicians can measure the superheat condition anywhere along this path. Two common locations are at the outlet of the evaporator and the inlet of the compressor. When measured at the outlet of the evaporator, it’s typically referred to as “evaporator superheat,” and when measured at the inlet of the compressor, it’s typically referred to as “total system superheat.” Refrigerant superheat is also occasionally measured at the outlet of the compressor, which is referred to as “compressor superheat.”
So, what does measuring superheat tell you about the system?
Measuring the evaporator’s superheat value is an important part of analyzing a system’s performance. If a lower-than-normal value is measured, too much refrigerant is entering the evaporator for the heat load. Many technicians refer to this as a “flooded evaporator.” If a higher-than-normal value is measured, too little refrigerant is entering the evaporator for the heat load. Technicians generally refer to this as a “starved evaporator.”
Part of effectively troubleshooting a mechanical refrigeration system should always include looking at the performance of the evaporator. This means measuring the evaporator’s superheat value. If this is overlooked, the true system problem can be misdiagnosed. For example, a system with a lower-than-normal suction pressure can be the result of several problems, two of which are a low refrigerant charge and a lack of airflow across the evaporator. The difference between these two problems can be seen by looking at the evaporator’s superheat. A low refrigerant charge will generally have a higher-than-normal evaporator superheat value, and lack of airflow across the evaporator will generally cause the evaporator’s superheat to be lower than normal.
What is the correct superheat value of the refrigerant leaving an evaporator? It is based on the system’s application and the manufacturer’s design. As a rule of thumb, medium-temperature systems will generally have a superheat of 8°-10°, and low-temperature applications, 4°-6°. But, again, always check with the system’s manufacturer for the recommended values.
Measuring the refrigerant superheat value at the entrance of the compressor is a means of ensuring no liquid is returning to the compressor. A refrigerant with a 0° superheat value (saturated refrigerant) at this location indicates that some amount of liquid refrigerant is returning to the compressor. At this condition, the amount of liquid refrigerant returning is unknown; however, any amount is too much. Liquid refrigerant returning to a compressor can damage a compressor in one of two ways.
It can mix with the compressor’s lubricating oil. As the liquid refrigerant comes in contact with the bearing surfaces it will wash away the oil film, causing wearing of the bearings. Depending on the percentage of liquid refrigerant to oil, the bearing wear could be mild to severe.
If a sufficient amount of refrigerant returns to the compressor, it may be possible for liquid to enter the cylinder(s) of the compressor and cause further damage to the compressor as it attempts to compress a liquid.
A very low superheat value could also lead to a system problem. Even though liquid refrigerant may not currently be returning to the compressor, it may be possible that, during low-load conditions, the superheat value of the refrigerant will drop to 0° and some amount
of liquid refrigerant will return to the compressor.
When analyzing mechanical refrigeration systems, technicians should remember to include measuring the refrigerant’s superheat value to help determine the root cause of a system problem.
Publication date: 3/2/2015